I believe contamination picked up on a drive train is being constantly displaced by the moving parts. Just as lubricant is being displaced from between roller and pin and side plates. All the debris and lubricant is looking for a resting place. It’s also being displaced from between the gear and chain. I don’t believe debris works its way into the chain, just as lubricant doesn’t. I think the contamination theory is overblown at the moment. I believe metal on metal wear is the culprit and the side to side movement at the rear gear cogs magnifies the issue. Therefore a lubricant with high psi capabilities and ability to flow would and is my choice, at the moment.